Ultra-low power multi-threshold asynchronous circuit design

ABSTRACT

A multi-threshold null convention logic circuit. The circuit includes a first circuit, a first high-threshold transistor coupled to V cc , and an inverter receiving power from the first high-threshold transistor, driven by the first circuit, and including an output.

RELATED APPLICATION

The present patent application is a continuation of co-pending U.S.patent application Ser. No. 13/175,168, filed Jul. 1, 2011 which is adivisional of U.S. patent application Ser. No. 12/771,886, filed Apr.30, 2010, now U.S. Pat. No. 7,977,972 which claims the benefit of U.S.Provisional Patent Application No. 61/232,181, filed on Aug. 7, 2009,the entire content of each are hereby incorporated by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under W15P7T-08-C-V404awarded by the DARPA Microsystems Technology Office. The government hascertain rights in the invention.

BACKGROUND

With the current trend of semiconductor devices scaling into the deepsubmicron region, design challenges that were previously minor issueshave now become increasingly important. Where in the past, dynamic,switching power has been the predominant factor in CMOS digital circuitpower dissipation, recently, with the dramatic decrease of supply andthreshold voltages, a significant growth in leakage power demands newdesign methodologies for digital integrated circuits (ICs). The maincomponent of leakage power is sub-threshold leakage, caused by currentflowing through a transistor even if it is supposedly turned off.Sub-threshold leakage increases exponentially with decreasing transistorfeature size.

Among the many techniques proposed to control or minimize leakage powerin deep submicron technology, Multi-Threshold CMOS (MTCMOS), whichreduces leakage power by disconnecting the power supply from the circuitduring idle (or sleep) mode while maintaining high performance in activemode, is very promising. MTCMOS incorporates transistors with two ormore different threshold voltages (Vt) in a circuit. Low-Vt transistorsoffer fast speed but have high leakage, whereas high-Vt transistors havereduced speed but far less leakage current. MTCMOS combines these twotypes of transistors by utilizing low-Vt transistors for circuitswitching to preserve performance and high-Vt transistors to gate thecircuit power supply to significantly decrease sub-threshold leakage.

There are multiple ways to implement MTCMOS in synchronous circuits. Onemethod is to use low-Vt transistors for critical paths to maintain highperformance, while using slower high-Vt transistors for the non-criticalpaths to reduce leakage. Besides this path replacement methodology,there are two other architectures for implementing MTCMOS. Acoarse-grained technique uses low-Vt logic for all circuit functions andgates the power to entire logic blocks with high-Vt sleep transistors,as shown in FIG. 1. The sleep transistors are controlled by a Sleepsignal. During active mode, the Sleep signal is deasserted, causing bothhigh-Vt transistors to turn on and provide a virtual power and ground tothe low-Vt logic. When the circuit is idle, the Sleep signal isasserted, forcing both high-Vt transistors to turn off and disconnectpower from the low-Vt logic, resulting in a very low sub-thresholdleakage current. One major drawback of this method is that partitioningthe circuit into appropriate logic blocks and sleep transistor sizing isdifficult for large circuits. An alternative fine-grained architecture,shown in FIG. 2, incorporates the MTCMOS technique within every gate,using low-Vt transistors for the Pull-Up Network (PUN) and Pull-DownNetwork (PDN) and a high-Vt transistor to gate the leakage currentbetween the two networks. Two additional low-Vt transistors are includedin parallel with the PUN and PDN to maintain nearly equivalent voltagepotential across these networks during sleep mode. Implementing MTCMOSwithin each gate solves the problems of logic block partitioning andsleep transistor sizing; however, this results in a large area overhead.

In general, three serious drawbacks hinder the widespread usage ofMTCMOS in synchronous circuits: 1) the generation of Sleep signals istiming critical, often requiring complex logic circuits; 2) synchronousstorage elements lose data when the power transistors are turned offduring sleep mode; and 3) logic block partitioning and transistor sizingis very difficult for the coarse-grained approach, which is critical forcorrect circuit operation, and the fine-grained approach requires alarge area overhead.

SUMMARY

The invention pertains to the fields of Computer Engineering andElectrical Engineering. The invention combines Multi-Threshold CMOS(MTCMOS) for synchronous circuits and asynchronous, delay-insensitiveNULL Convention Logic (NCL), to yield an ultra-low power asynchronouscircuit design methodology, named Multi-Threshold NULL Convention Logic(MTNCL).

In one embodiment, the invention provides a delay-insensitive fixedearly-completion input-incomplete architecture (FECII). The FECIIarchitecture includes MTNCL combinational logic blocks, which are builtusing MTNCL logic gates. These gates have a plurality of low-Vttransistors having high speed and high leakage current, and a pluralityof high-Vt transistors having lower speed and low leakage current. Thehigh-Vt transistors gate power to the low-Vt transistors when the MTNCLcircuit is in a sleep mode. The low-Vt transistors are used forswitching the output in active mode. Due to the removal of hysteresis,most MTNCL gates have fewer transistors compared to their regular NCLcounterparts, thereby eliminating the area overhead. The FECIIarchitecture further includes a plurality of delay-insensitiveearly-completion registers, each register coupled to a respective MTNCLlogic block to form a pipeline stage. The completion detection signal,Ko, of each pipeline stage, and the handshaking signal from the nextstage, generate the handshaking signal for the current stage, which isused as both the inversed Ki signal to the register of the current stageand the Sleep control signal of the next stage.

In another embodiment the invention provides an early-completioninput-incomplete MTNCL architecture (ECII). While similar to those usedin FECII architecture, the MTNCL logic gates used in ECII architectureinclude hold1 structures, in order to avoid partial NULL from passingthrough since it may cause the loss of delay-insensitivity in ECIIarchitecture.

MTNCL eliminates the three drawbacks of using the low-power MTCMOStechnique with synchronous circuits (i.e., 1) the generation of Sleepsignals is timing critical, often requiring complex logic circuits; 2)synchronous storage elements lose data when the power transistors areturned off during sleep mode; and 3) logic block partitioning andtransistor sizing is very difficult for the coarse-grained approach,which is critical for correct circuit operation, and the fine-grainedapproach requires a large area overhead), to yield an ultra-low powerasynchronous circuit design methodology. MTNCL circuits have beendemonstrated to dissipate 99% less static power compared to regular NCLcircuits, and 38% less static power compared to MTCMOS synchronouscircuits; MTNCL circuits have been demonstrated to utilize 62% lessenergy per operation compared to regular NCL circuits, and 99% lessenergy per operation compared to MTCMOS synchronous circuits; and MTNCLcircuits are slightly faster than their regular NCL counterparts, andhave comparable speed to synchronous circuits. Additionally, since MTNCLcircuits are delay-insensitive, the supply voltage can be dropped toeven sub-threshold levels to substantially further decrease power usage,at the cost of decreased performance, without requiring any circuitmodifications. MTNCL circuits designed with the 1.2V IBM 8RF 130 nmprocess have been demonstrated to work correctly down to a sub-thresholdsupply voltage of about 0.1V, and an energy-optimal supply voltage ofabout 0.3V, where energy per operation is reduced by a factor of about400× and static power is reduced by a factor of about 15×.

In another embodiment, the invention provides an early completion inputincomplete asynchronous circuit (ECII). The ECII includes a delayinsensitive register, a multi-threshold null convention logic circuit,and an early completion circuit. The delay insensitive register has adata input, a data output, a request input, and an acknowledge output.The multi-threshold null convention logic circuit has a data inputcoupled to the delay insensitive register data output, a sleep input,and an output coupled to a data input of a delay insensitive register ofa subsequent ECII. The early completion circuit has a first inputcoupled to the acknowledge output, a second input coupled to the requestinput, and an output coupled to the sleep input and to a request inputof a delay insensitive register of a previous ECII.

In another embodiment, the invention provides a fixed early completioninput incomplete asynchronous circuit (FECII). The FECII includes adelay insensitive register, a multi-threshold null convention logiccircuit, an early completion circuit, and an inverter. The delayinsensitive register has a data input, a data output, a request input,and an acknowledge output. The multi-threshold null convention logiccircuit has a data input coupled to the delay insensitive register dataoutput, a sleep input, and an output coupled to a data input of a delayinsensitive register of a subsequent FECII. The early completion circuithas a first input coupled to the acknowledge output, a second inputcoupled to an output of an early completion circuit of a subsequentFECII, and an output coupled to the sleep input and to an input of anearly completion circuit of a previous FECII. The inverter is coupled tothe output of the early completion circuit and has an output coupled tothe request input.

In another embodiment, the invention provides a early completion inputincomplete asynchronous circuit (ECII). The circuit includes a delayinsensitive register, a multi-threshold null convention logic circuit,and a slept early completion circuit. The delay insensitive register hasa data input and a data output. The multi-threshold null conventionlogic circuit has a data input coupled to the delay insensitive registerdata output, a sleep input, and a data output. The slept earlycompletion circuit has a first input coupled to an output of an earlycompletion circuit of a subsequent ECII, a second input coupled to theoutput of an early completion circuit of a previous ECII, a third inputcoupled to the delay insensitive register data input, and an outputcoupled to the sleep input.

In another embodiment, the invention provides a multi-threshold nullconvention logic circuit. The circuit includes a first circuit coupledto V_(cc), a set circuit coupled to the first circuit, a firsthigh-threshold transistor coupled between the set circuit and ground, asecond high-threshold transistor coupled to V_(cc), a firstlow-threshold transistor coupled to the second high-thresholdtransistor, a third high-threshold transistor coupled between the firstlow-threshold transistor and ground, a second low-threshold transistorcoupled between the coupling of the third high-threshold transistor andthe first low-threshold transistor and ground, and an output coupled tothe coupling between the third high-threshold transistor, the firstlow-threshold transistor, and the second low-threshold transistor. Thefirst high-threshold transistor is driven by a SLEEP signal. The secondhigh-threshold transistor is driven by a SLEEP signal. The firstlow-threshold transistor is driven by the coupling between the firstcircuit and the set circuit. The third high-threshold transistor isdriven by the coupling between the first circuit and the set circuit.The second low-threshold transistor is driven by the SLEEP signal.

In another embodiment, the invention provides a multi-threshold nullconvention logic circuit. The circuit includes a first circuit, a firsthigh-threshold transistor coupled to Vcc, and an inverter receivingpower from the first high-threshold transistor, driven by the firstcircuit, and including an output.

In another embodiment, the invention provides a multi-threshold nullconvention logic circuit. The circuit includes an inverter coupled toground and having an output, a first circuit driving the inverter; and ahigh-threshold transistor driven by a sleep signal having an assertedand a non-asserted state. The high-threshold transistor couples theinverter to Vcc when the sleep signal is in the non-asserted state andisolates the inverter from Vcc when the sleep signal is in the assertedstate. The first circuit also drives the inverter with a logic 1 duringa sleep mode when the sleep signal is asserted. The circuit prevents aglitch on the output when the sleep signal switches from the assertedstate to the non-asserted state.

In another embodiment, the invention provides a method of manufacturinga logic circuit having an output and receiving a sleep signal having anasserted state and a non-asserted state. The method includes providing afirst circuit for performing the logic, providing an inverter driven bythe second output, and providing a sleep circuit to couple and isolatethe inverter to Vcc and the first circuit to ground based on the stateof the sleep signal. The first circuit is coupled to Vcc by one or morehigh-threshold transistors, and has a second output. The inverter drivesthe output.

Other aspects of the invention will become apparent by consideration ofthe detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a prior art MTCMOS circuitarchitecture.

FIG. 2 is a schematic diagram of a prior art MTCMOS circuit as appliedto a Boolean gate.

FIGS. 3 a-3 d are block diagrams of a MTNCL pipeline architecture usingregular completion.

FIGS. 4 a-4 d are block diagrams of a MTNCL pipeline architecture usingearly-completion.

FIGS. 5 a and 5 b are schematic diagrams of original MTCMOS NCLthreshold gates.

FIG. 6 is a schematic diagram of an original MTCMOS static TH23 gate.

FIG. 7 a is a schematic diagram of a static TH23 NCL implementation.

FIG. 7 b is a schematic diagram of a semi-static TH23 NCLimplementation.

FIG. 8 a is schematic diagram of an original static MTCMOS NCL thresholdgate structure.

FIG. 8 b is a graph showing an output glitch produced by the gatestructure of FIG. 8 a.

FIG. 9 a is a schematic diagram of a SMTNCL gate structure.

FIG. 9 b is a schematic diagram of a SMTNCL TH23 implementation.

FIGS. 10 a-10 c are block diagrams of a ECII architecture using SMTNCLgates.

FIG. 11 is a block diagram of a fixed early-completion input-incompletearchitecture (FECII).

FIG. 12 a is a schematic diagram of a SMTNCL1 gate structure.

FIG. 12 b is a schematic diagram of a SMTNCL1 TH23 implementation.

FIG. 13 a is a schematic diagram of a SSMTNCL1 gate structure.

FIG. 13 b is a schematic diagram of a SSMTNCL1 TH23 implementation.

FIG. 14 a is a schematic diagram of a SSMTNCL gate structure.

FIG. 14 b is a schematic diagram of a SSMTNCL TH23 implementation.

FIG. 15 is a block diagram of a Slept Early Completion Input-Incomplete(SECII) architecture.

FIG. 16 is a schematic diagram of an Early completion component withSleep input.

FIG. 17 is a block diagram of a Slept Early Completion and RegistrationInput-Incomplete (SECRII) architecture.

FIG. 18 is a schematic diagram of a DI register with Sleep input.

DETAILED DESCRIPTION

Before any embodiments of the invention are explained in detail, it isto be understood that the invention is not limited in its application tothe details of construction and the arrangement of components set forthin the following description or illustrated in the following drawings.The invention is capable of other embodiments and of being practiced orof being carried out in various ways.

The three drawbacks of MTCMOS are eliminated by utilizing NULLConvention Logic (NCL) in conjunction with the MTCMOS technique, asdescribed in embodiments of this invention, yielding an ultra-low powerasynchronous circuit design methodology, referred to as Multi-ThresholdNULL Convention Logic (MTNCL).

Early-Completion Input-Incomplete (ECII) MTNCL Architecture

NCL threshold gates are larger and implement more complicated functionsthan basic Boolean gates, such that fewer threshold gates are normallyneeded to implement an arbitrary function compared to the number ofBoolean gates; however, the NCL implementation often requires moretransistors. Therefore, incorporating MTCMOS inside each threshold gatewill likely facilitate easy sleep transistor sizing without requiring aslarge of an area overhead. Since floating nodes may result insubstantial short circuit power consumption at the following stage, anMTCMOS structure similar to the one shown in FIG. 2 is used to pull theoutput node to ground during sleep mode. All of the MTNCL gates in apipeline stage being in sleep mode (i.e., all gate outputs are logic 0)is equivalent to the pipeline stage being in the NULL state. Hence,after each DATA cycle, all MTNCL gates in a pipeline stage can be forcedto output logic 0. By asserting the sleep control signal, instead ofpropagating a NULL wavefront through the stage, data is not lost duringsleep mode.

Since a completion detection signal, Ko, indicates whether thecorresponding pipeline stage is ready to undergo a DATA or NULL cycle,Ko can be used as a sleep control signal without requiring anyadditional hardware. In contrast, complex Sleep signal generationcircuitry is needed for synchronous MTCMOS circuits. However, directlyimplementing this idea, using regular NCL completion, compromisesdelay-insensitivity.

Referring to FIG. 3, each inverted completion signal is used as thesleep signal for all MTNCL gates in the corresponding pipeline stage.Looking at the left stage, after a DATA (D) cycle, the completion signalbecomes request for null (rfn) (i.e., logic 0), which forces allthreshold gates in the stage to enter sleep mode, since the next cyclewill be NULL (N). When this sleep generated NULL wavefront is latched bythe subsequent register, the stage's completion signal switches back torequest for data (rfd) (i.e., logic 1). If this occurs before all bitsof the preceding DATA wavefront become NULL, the non-NULL precedingwavefront bits will be retained and utilized in the subsequentoperation, thereby compromising delay-insensitivity.

Early-completion can be used in lieu of regular completion to solve thisproblem. With early-completion, the register inputs are used, instead ofthe outputs, along with the previous stage's completion signal togenerate the current stage's completion signal. As shown in FIG. 4, eachcompletion signal is used as the sleep signal for all threshold gates inthe subsequent pipeline stage. Now the combinational logic won't be putto sleep until all inputs are NULL and the stage is requesting NULL;therefore the NULL wavefront is ready to propagate through the stage, sothe stage can be put to sleep without compromising delay-insensitivity.The stage will then remain in sleep mode until all inputs are DATA, andthe stage is requesting DATA (i.e., the stage is ready to evaluate). Theearly-completion MTNCL architecture ensures input-completeness throughthe sleep mechanism (i.e., the circuit is only put to sleep after allinputs are NULL, and only evaluates after all inputs are DATA). Usinginput-incomplete logic functions to design the circuit decreases areaand power and increases speed.

MTNCL Threshold Gate Design for ECH Architecture

Incorporating MTCMOS structure inside each NCL threshold gate, resultsin a number of the original transistors no longer being needed. Forexample, as shown in FIG. 5 a, the reset circuitry is no longer needed,because the gate output is forced to NULL by the MTCMOS sleep mechanism(instead of all inputs becoming logic 0). Hold1 was used to ensure thatthe gate remains asserted, once it has become asserted, until all inputsare deasserted. This guarantees input-completeness with respect to theNULL wavefront. However, since the ECII architecture guaranteesinput-completeness through the sleep mechanism, NCL gate hysteresis isno longer required. Hence, the hold1 circuitry, and corresponding NMOStransistor, are removed. The PMOS transistor is also removed to maintainthe complementary nature of CMOS logic (i.e., set and hold0 arecomplements of each other), such that the gate does not float.

A direct MTCMOS NCL threshold gate implementation, similar to thestructure shown in FIG. 2, is shown in FIG. 5 b. All PMOS transistors,except the inverter, are high-Vt, (denoted by a dotted circle). This ispossible because they are only turned on when the gate enters sleep modeand the inputs become logic 0, and they remain on when the gate exitssleep mode, until the gate's set condition becomes true. In both cases,the gate output is already logic 0; therefore, the speed of these PMOStransistors does not affect performance, and high-Vt transistors areused to reduce leakage current. During active mode, the Sleep signal islogic 0 and Sleep is logic 1, such that sleep transistors M1 and M2 areturned on, and bypass transistors M3 and M4 and the output pull-downtransistor M5 are turned off, and the gate functions as normal. Duringsleep mode, Sleep is logic 1 and Sleep is logic 0, such that M5, whichis a low-Vt transistor, is turned on, quickly pulling the output tologic 0, while M3 and M4 are turned on to minimize the voltage potentialacross the hold0 and set blocks, respectively. The high-Vt gatingtransistors, M1 and M2, are turned off to reduce leakage. An example ofan MTNCL implementation of a static TH23 gate is shown in FIG. 6,whereas an original static TH23 gate is shown in FIG. 7 a.

Note that the MTNCL TH23 gate is actually smaller than the original TH23gate (i.e., 17 vs. 18 transistors). Although the MTNCL structure addsfive transistors to each gate, this only increases total number oftransistors for 3 of the 27 threshold gates, since the reset and hold1blocks are removed. This results in the total number of transistors inthe MTNCL version normally being less than the original version.Exceptions include the three TH1n gates, which are OR gates, andtherefore do not have extra hysteresis circuitry (i.e., reset=hold0 andset=hold1), no transistors are removed for the MTNCL versions.Therefore, these three MTNCL gates require 5 additional transistorseach.

This initial MTNCL static threshold gate structure has been used toimplement an 8-bit×8-bit pipelined array multiplier using a 1.2V 130 nmIBM 8RF CMOS process, reducing leakage power 150 times and reducingactive energy by 1.8 times compared to a regular NCL low-Vt counterpart.However, as shown in FIG. 8 b, this structure produces unwanted glitchesat the gate outputs. Referring to FIG. 8 a, during sleep mode, Sleep islogic 1 and Sleep is logic 0. Q1 and Q5 are off, while Q1, Q2, and Q4are on. The internal parasitic capacitance, Cp, is discharged throughQ4, making the internal node, p, logic 0. When the gate is taken out ofsleep mode, Sleep is logic 0 and Sleep is logic 1, Q1 and Q5 are on, andQ2, Q3, and Q4 are off. Since all inputs are logic 0 at this moment, dueto the preceding NULL/sleep cycle, Cp begins charging through the PMOSnetwork and Q1. However, before the voltage on p rises toV_(DD)−|V_(TP)| (where V_(TP) is the threshold voltage of the PMOStransistor in the output inverter), the gate output (i.e., the inputsignal to the inverter and the voltage on Cp) starts to rise,momentarily turning on the PMOS transistor, and causing a glitch shownin FIG. 8 b. With a supply voltage of 1.2V, these glitches can be ashigh as 400 mV, and are able to propagate through logic gates. Althougha multiplier test circuit still functioned correctly, these glitchesneed to be removed to ensure reliable operation and eliminate glitchpower.

To reduce or eliminate the glitch, the MTNCL threshold gate structure ismodified, as shown in FIG. 9, by moving the power gating high-Vttransistor to the PDN such that during sleep mode the internal node ischarged to logic 1. The two bypass transistors, Q3 and Q4, which haveonly a very minimal contribution to leakage savings can be removed toreduce the area of the gate. Therefore, when the gate is taken out ofsleep mode, the output will remain at logic 0 without any glitch, due tothe internal logic 1 flowing through the output inverter, until the DATAwavefront arrives. Since the internal node is logic 1 during sleep mode,and the output is logic 0, the NMOS transistor in the output inverter isno longer on the critical path and can be a high-Vt transistor. Thismodified Static MTNCL threshold gate structure is referred to as SMTNCL.

Delay-Insensitivity Analysis

Combining the ECII architecture with the SMTNCL gate structure, resultsin a delay-sensitivity problem after a DATA cycle, if most, but not all,inputs become NULL (see FIG. 10). Because the subsequent stage isrequesting NULL, and because the hold 1 logic has been removed from theSMTNCL gates, this Partial NULL (PN) wavefront can pass through thestage's input register causing all stage outputs to become NULL, beforeall inputs are NULL and the stage is put to sleep. This violates aninput-completeness criteria that requires that all outputs of acombinational circuit to not transition from NULL to DATA until allinputs have transitioned from NULL to DATA, and that all outputs of acombinational circuit may not transition from DATA to NULL until allinputs have transitioned from DATA to NULL. This can cause thesubsequent stage to request the next DATA while the previous stage inputis still a partial NULL, resulting in the preceding wavefront bits thatare still DATA being retained and utilized in the subsequent operation.This can compromise delay-insensitivity, similar to the problemencountered using regular completion, as explained above.

There are at least two solutions to this problem, one at thearchitecture level and the other at the gate level. Since the problem iscaused by a partial NULL passing through the register, this can be fixedat the architecture-level by ensuring that the NULL wavefront is onlyallowed to pass through the register after all register inputs are NULL.This is achievable by a Fixed Early-Completion Input-Incomplete (FECII)architecture which uses the stage's inverted sleep signal as its inputregister's Ki signal. The FECII architecture is shown in FIG. 11.Compared to ECII, FECII is slower because the registers must wait untilall inputs become DATA/NULL before they are latched. Note that becausethe stage will remain in sleep mode until all inputs are DATA, a partialDATA wavefront passing through the register does not pose a problem, andall stage outputs will remain NULL until all inputs are DATA.

This problem can be solved at the gate level using a Static MTNCLimplementation with hold1 (SMTNCL1), which requires 2 additionaltransistors vs. the SMTNCL gate. The feedback NMOS transistor issufficient to hold the output at logic 1, without requiring thetraditional NCL hold1 circuitry, because this ensures that once the gateoutput has been asserted due to the current DATA wavefront, that it willonly be deasserted when the gate is put to sleep (i.e., when all circuitinputs are NULL), and will not be deasserted due to a partial NULLwavefront. The SMTNCL1 design is shown in FIG. 12.

The MTCMOS structure can also be applied to semi-static NCL gates, whichutilize a weak feedback inverter to implement the hold1 and hold0functions. This Semi-Static MTNCL design with hold1 (SSMTNCL1), is shownin FIG. 13. Note that a 1 (one) at the end of an MTNCL gate name denotesthat the gate includes hold1 circuitry. The NMOS transistor in the weakinverter that serves as the hold1 function is not needed for the FECIIarchitecture; and can be removed to save area. This results in amodified Semi-Static MTNCL design (SSMTNCL) shown in FIG. 14.

To summarize, the ECII architecture works with SMTNCL1 or SSMTNCL1gates, which include the hold1 function. The FECII architecture workswith all four MTNCL gate designs (i.e., SMTNCL, SMTNCL1, SSMTNCL1, andSSMTNCL). However, the SMTNCL and SSMTNCL gates require fewertransistors than their equivalent SMTNCL1 and SSMTNCL1 gates, such thatthe FECII architecture would normally use either the SMTNCL or SSMTNCLgates. Additionally, the ECII architecture is faster than FECII; and thestatic NCL gates (i.e., SMTNCL and SMTNCL1) perform better than theirsemi-static counterparts (i.e., SSMTNCL and SSMTNCL1) at reduced supplyvoltages. This is because the static implementations do not utilize aweak inverter, which ceases to operate properly with a substantiallyreduced supply voltage, for state-holding.

In the MTNCL architectures described so far, an NCL circuit's C/L wasslept in lieu of the NULL cycle to significantly reduce leakage power.However, during sleep mode the circuit's completion and registrationlogic remains active, which for a fine grain pipelined circuit may be asignificant portion of the logic. Therefore, it would be very beneficialto be able to sleep the completion and registration logic in addition tothe C/L. The completion logic can be slept by modifying the ECIIarchitecture, shown in FIG. 4, to include a sleep input to thecompletion logic and use SMTNCL1 gates to implement the completionlogic, as shown in FIGS. 15 and 16, respectively. Note that the finalinverting TH22 gate is a regular NCL gate, which is not slept. This isconsistent with the NULL cycle, where the internal completion componentgates are all logic 0, except for the final inverting TH22 gate.

During a NULL cycle, the register output is also NULL, so it too can beslept, as shown in FIG. 17. Instead of using two SMTNCL1 TH22 gates toimplement the register, the sleep transistors for each rail can becombined, such that a dual-rail register is implemented as a singlecomponent in order to reduce area, as shown in FIG. 18. Note that thisarchitecture is similar to the FECII architecture shown in FIG. 11,which does not allow a partial NULL to propagate through the register,such that the C/L can be implemented with the smaller SMTNCL gatesinstead of SMTNCL1 gates.

Table I compares the various MTNCL architectures and original NCLcircuits using a 4-stage pipelined IEEE single-precision floating-pointco-processor, which performs addition, subtraction, and multiplication.

TABLE I MTNCL comparisons. # Tran- T_(DD) Energy/ Leakage sistors (ns)Operation (pJ) Power (nW) NCL Low-V_(t) 158059 14.1 25.6 13300 NCLHigh-V_(t) 158059 32.5 25.9 300 SMTCNL with FECII 111506 13.4 20.7 377SMTNCL1 with ECII 119706 14.3 20.6 382 SMTNCL1 with SECII 119244 12.820.5 382 SMTNCL with SECRII 96640 12.6 19.5 346

Some concepts of early completion circuits are described in S.C. Smith,“Speedup of Self-Timed Digital Systems Using Early Completion,” The IEEEComputer Society Annual Symposium on VLSI, pp. 107-113, April 2002, thecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A multi-threshold null convention logic circuit,the circuit comprising: a first circuit; a first high-thresholdtransistor coupled to V_(cc); and an inverter receiving power from thefirst high-threshold transistor, driven by the first circuit, andincluding an output; wherein the set circuit is isolated from groundduring a sleep cycle.
 2. The circuit of claim 1, wherein the firsthigh-threshold transistor is driven by a sleep signal.
 3. The circuit ofclaim 1, wherein a second high-threshold transistor couples the firstcircuit to ground and is driven by a sleep signal.
 4. The circuit ofclaim 1, wherein a first low-threshold transistor couples the inverteroutput to ground and is driven by a sleep signal.
 5. The circuit ofclaim 1, wherein the output is driven to ground during a sleep cycle. 6.The circuit of claim 1, wherein the first circuit includes a hold0circuit and a set circuit coupled together.
 7. The circuit of claim 1,wherein the first circuit drives the inverter with a logic 1 during asleep mode following assertion of the sleep signal.
 8. The circuit ofclaim 7, wherein isolating the inverter from Vcc and driving theinverter with a logic 1 during the sleep cycle prevents a glitch uponexiting the sleep cycle
 10. 9. The circuit of claim 1, wherein theinverter is isolated from Vcc during a sleep cycle.
 10. The circuit ofclaim 1, wherein the first circuit is coupled to Vcc exclusively by oneor more high-threshold transistors.
 11. A multi-threshold nullconvention logic circuit, the circuit comprising: an inverter coupled toground and having an output; a first circuit driving the inverter; and ahigh-threshold transistor driven by a sleep signal having an assertedand a non-asserted state, and coupling the inverter to Vcc when thesleep signal is in the non-asserted state and isolating the inverterfrom Vcc when the sleep signal is in the asserted state, and a secondhigh-threshold transistor driven by a second sleep signal having asecond asserted stated and a second non-asserted state, the secondhigh-threshold transistor coupling the first circuit to ground when thesecond sleep signal is in the second asserted state and isolating thefirst circuit from ground when the second sleep signal is in the secondnon-asserted state; wherein the first circuit drives the inverter with alogic 1 during a sleep mode following assertion of the sleep signal;wherein a glitch on the output when the sleep signal switches from theasserted state to the non-asserted state is prevented.
 12. The circuitof claim 11, wherein the first circuit includes a plurality ofhigh-threshold transistors and low-threshold transistors, the firstcircuit coupled to Vcc by only high-threshold transistors.
 13. Thecircuit of claim 11, wherein the second signal is in the secondnon-asserted state when the sleep signal is in the asserted state and isin the second asserted state when the sleep signal is in thenon-asserted state.
 14. A method of manufacturing a logic circuit havingan output and receiving a sleep signal having an asserted state and anon-asserted state, the method comprising: providing a first circuit forperforming the logic, the first circuit coupled to Vcc by one or morehigh-threshold transistors, the first circuit having a second output;providing an inverter driven by the second output, the inverter drivingthe output; and providing a sleep circuit to couple and isolate theinverter to Vcc and the first circuit to ground based on the state ofthe sleep signal; and isolating the first circuit from ground via thehigh-threshold transistor of the sleep circuit when the sleep signal isin the asserted state.
 15. The method of claim 14, further comprisingcoupling the inverter to Vcc via a high-threshold transistor of thesleep circuit when the sleep signal is in the non-asserted state. 16.The method of claim 14, further comprising isolating the inverter fromVcc via the high-threshold transistor of the sleep circuit when thesleep signal is in the asserted state.
 17. The method of claim 14,further comprising coupling the first circuit to ground via ahigh-threshold transistor of the sleep circuit when the sleep signal isin the non-asserted state.
 18. The method of claim 14, furthercomprising coupling the output to ground via a low-threshold transistorwhen the sleep signal is in the asserted state.
 19. The method of claim14, wherein the logic circuit is a threshold gate.